1. Field of the Invention
This invention relates to molecular models for simulating the structure of the nucleic acids, DNA and RNA.
One of the great scientific advances of the mid-twentieth century has been the discovery of the structure of the nucleic acids which make up the portions of cells which determine physical characteristics of living organisms. According to presently accepted theory, the chromosomes found in the nucleus of cells contain within them genes which determine the physical characteristics of all living organisms. The genes are composed of long chain molecules of nucleic acid which comprise two major types, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). DNA is found primarily within the chromosomes whereas most of the RNA is located outside the nucleus in the cytoplasm.
The accepted structure of the DNA molecule is that proposed by Watson and Crick. According to the Watson-Crick model, DNA is comprised of two intertwining strands forming an interlocking double helix orientated about a common central axis. The strands are composed of alternating units of a sugar (deoxyribose) and a phosphate linked together by chemical bases attached to the sugar units. The bases are made up of the nitrogen-containing compounds purine and pyrimidine--the purines being adenine and guanine and the pyrimidines being cytosine and thymine. A molecular group consisting of a sugar unit having a phosphate unit attached to one side and a purine or pyrimidine compound to the other side is called a nucleotide.
While there are four bases in the DNA molecule, it can be shown that only purine-pyrimidine bonds are possible and that purine-purine or pyrimidine-pyrimidine bonds are theoretically impossible. In fact, it has been found an adenine is always joined to a thymine by a hydrogen bond and similarly, a guanine is always connected to a cytosine by a hydrogen bond. Thus, the two halves of the DNA molecule are complimentarywhere a nucleotide in one half contains adenine the other half will have thymine and where the first half contains guanine the other half will have cytosine. It is the order in which these pairs of bases are arranged in the DNA molecule that determines the genetic code.
When a cell divides, the DNA molecules making up the chromosomes replicate themselves by a process in which each half acts as a model for the new molecule. During division, the double helix splits at the purine-pyrimidine hydrogen bonds and free nucleotides (which are always present in the cell) join each half. The free nucleotides couple to the nucleotides in the splitting DNA molecule in such a way that only adeninethymine and guanine-cytosine bonds are formed. In addition, the sugar and phosphate units of the free nucleotides are joined together by covalent bonds once they have been positioned along the DNA chain, this reaction being catalyzed by an enzyme. As a result, two new DNA molecules are formed which are identical with the original molecule.
As previously mentioned, ribonucleic acid (RNA) also exists within the cell. RNA is quite similar to DNA except that the sugar deoxyribose is replaced by the sugar ribose and the base thymine is replaced by another pyrimidine, uracil. One form of RNA, termed "messenger-RNA", is formed within the nucleus by a replication process similar to that which the DNA molecule is caused to split. The nucleotides in the messenger-RNA correspond to those in the DNA except for the substitution of uracil for thymine and the additional atom of oxygen in the sugar. The resultant messenger-RNA molecule, therefore, carries the same genetic code as the gene that formed it.
After it is formed, the messenger-RNA molecule breaks out of the nucleus and moves into the cytoplasm where it attaches itself to a ribosome. A ribosome is a particle found in the cytoplasm which is made up of about half RNA and half protein. The messenger-RNA is now in a position to direct the synthesis of protein by joining a number amino acids to form a polypeptide chain. This is accomplished with the aid of another form of RNA, referred to as "transfer-RNA", which is small enough to be readily soluble in the cell fluid. There are a number of variations of transfer-RNA and each has the property that it will attach itself to a specific amino acid. In addition, each form of transfer-RNA has three bases from the group adenine, uracil, guanine and cytosine. The particular three bases comprising each transfer-RNA molecule corresponds to the specific amino acid with which a transfer-RNA molecule is associated.
After attaching to an amino acid, each transfer-RNA molecule migrates to a location on the messenger-RNA molecule having a base sequence corresponding to the compliment of the triplet code on the transfer-RNA. When all the transfer-RNA molecules are in place along the polynucleotide chain of the messenger-RNA, the amino acids are in the correct order for enzymatic processes to bring about a reaction that combines them into a specific polypeptide chain corresponding to the desired protein.
From this brief summary, it can be seen that the molecular configurations of the nucleic acids and the processes involved in the formation of protein are not only quite complex, but are three dimensional in nature.
2. Brief Description of the Prior Art
When the structure of DNA and RNA was first announced, it was illustrated for classroom purposes only by lecture and two dimensional drawings.
Very complex models of DNA and RNA have been constructed using individual pieces simulating the various atoms which make up the molecule. A model of this type is extremely complex, difficult to assemble, and very expensive.
Klotz U.S. Pat. No. 3,296,714 discloses a model for nucleic acids such as DNA and RNA in which the individual nucleotides are illustrated by thin tubular members which are assembled in a ladder-shaped structure and then twisted into helical form. This model has the disadvantage that the bases of each of the nucleotides do not have a shape corresponding to the known structure of the nucleotides and the model is not self-supporting and does not illustrate adequately the different bases forming the several nucleotides making up the DNA or RNA molecules.
Baker U.S. Pat. No. 3,594,924 discloses a model for DNA or RNA in which the sugar, phosphate, and bases are illustrated by beads of varying shape which are connected end to end or side to side, in the case of the bases, and twisted into a helical form and supported on a supporting rod. This model is complex to assemble and has the disadvantage that the individual beads do not approximate the proportions and shape of the components of the DNA or RNA helixes.